As a method for producing a glass particle deposit, there is known a method (multilayer deposition method) of depositing glass particles on an outside of a starting rod stratiformly to obtain a glass particle deposit as disclosed in U.S. Pat. No. 5,116,400.
According to the method for producing a glass particle deposit, a starting position of traverse is moved to a predetermined position for every traverse movement and, after the starting position of traverse reaches a predetermined position, the position is moved reversely back to the initial traverse starting position. As a result, variation in an end portion of traverse in which the time required for soot deposition is substantially long or variation of a degree of touching of a burner flame against the glass particle deposit, or the like is distributed into the whole glass particle deposit. The time and atmosphere substantially required for depositing glass particles are averaged evenly throughout the glass particle deposit. In this manner, the amount of deposited glass particles is made uniform in the longitudinal direction to thereby reduce outer diameter variation. Moreover, when glass particles are deposited while the starting position of traverse is moved, clean air is supplied to the whole glass particle deposit perpendicularly to the longitudinal direction so that the deposition temperature gradient in the longitudinal direction of the glass particle deposit is reduced.
Incidentally, in the production of the glass particle deposit, a surface of the glass particle deposit is cooled by a flow of clean gas (CG) introduced into a glass particle deposit producing apparatus or by surplus air or the like entered in through a gap formed around a support rod or the like. It has been found that bulk density consequently varies in the longitudinal direction of the glass particle deposit to cause outer diameter variation. That is, the amount of deposited glass particles increases in a portion of low bulk density having a large diameter and, conversely, the amount of deposited glass particles decreases in a portion of high bulk density having a small diameter because of variation in bulk density in the longitudinal direction. For this reason, the thickness of glass in a glass preform finally obtained also varies in the longitudinal direction to cause deterioration of quality.